This document seeks to dispel the mystery and confusion surrounding LLVM's
GetElementPtr (GEP) instruction. Questions about the wiley GEP instruction are
probably the most frequently occuring questions once a developer gets down to
coding with LLVM. Here we lay out the sources of confusion and show that the
GEP instruction is really quite simple.

When people are first confronted with the GEP instruction, they tend to
relate it to known concepts from other programming paradigms, most notably C
array indexing and field selection. However, GEP is a little different and
this leads to the following questions, all of which are answered in the
following sections.

The confusion with the first index usually arises from thinking about
the GetElementPtr instruction as if it was a C index operator. They aren't the
same. For example, when we write, in "C":

AType* Foo;
...
X = &Foo->F;

it is natural to think that there is only one index, the selection of the
field F. However, in this example, Foo is a pointer. That
pointer must be indexed explicitly in LLVM. C, on the other hand, indexs
through it transparently. To arrive at the same address location as the C
code, you would provide the GEP instruction with two index operands. The
first operand indexes through the pointer; the second operand indexes the
field F of the structure, just as if you wrote:

X = &Foo[0].F;

Sometimes this question gets rephrased as:

Why is it okay to index through the first pointer, but
subsequent pointers won't be dereferenced?

The answer is simply because memory does not have to be accessed to
perform the computation. The first operand to the GEP instruction must be a
value of a pointer type. The value of the pointer is provided directly to
the GEP instruction as an operand without any need for accessing memory. It
must, therefore be indexed and requires an index operand. Consider this
example:

In this "C" example, the front end compiler (llvm-gcc) will generate three
GEP instructions for the three indices through "P" in the assignment
statement. The function argument P will be the first operand of each
of these GEP instructions. The second operand indexes through that pointer.
The third operand will be the field offset into the
struct munger_struct type, for either the f1 or
f2 field. So, in LLVM assembly the munge function looks
like:

These GEP instructions are simply making address computations from the
base address of MyVar. They compute, as follows (using C syntax):

idx1 = (char*) &MyVar + 0

idx2 = (char*) &MyVar + 4

idx3 = (char*) &MyVar + 8

Since the type i32 is known to be four bytes long, the indices
0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
memory is accessed to make these computations because the address of
%MyVar is passed directly to the GEP instructions.

The obtuse part of this example is in the cases of %idx2 and
%idx3. They result in the computation of addresses that point to
memory past the end of the %MyVar global, which is only one
i32 long, not three i32s long. While this is legal in LLVM,
it is inadvisable because any load or store with the pointer that results
from these GEP instructions would produce undefined results.

The GEP above yields an i32* by indexing the i32 typed
field of the structure %MyStruct. When people first look at it, they
wonder why the i64 0 index is needed. However, a closer inspection
of how globals and GEPs work reveals the need. Becoming aware of the following
facts will dispell the confusion:

The type of %MyStruct is not{ float*, i32 }
but rather { float*, i32 }*. That is, %MyStruct is a
pointer to a structure containing a pointer to a float and an
i32.

Point #1 is evidenced by noticing the type of the first operand of
the GEP instruction (%MyStruct) which is
{ float*, i32 }*.

The first index, i64 0 is required to step over the global
variable %MyStruct. Since the first argument to the GEP
instruction must always be a value of pointer type, the first index
steps through that pointer. A value of 0 means 0 elements offset from that
pointer.

The second index, i32 1 selects the second field of the
structure (the i32).

The GetElementPtr instruction dereferences nothing. That is, it doesn't
access memory in any way. That's what the Load and Store instructions are for.
GEP is only involved in the computation of addresses. For example, consider
this:

In this example, we have a global variable, %MyVar that is a
pointer to a structure containing a pointer to an array of 40 ints. The
GEP instruction seems to be accessing the 18th integer of the structure's
array of ints. However, this is actually an illegal GEP instruction. It
won't compile. The reason is that the pointer in the structure must
be dereferenced in order to index into the array of 40 ints. Since the
GEP instruction never accesses memory, it is illegal.

In order to access the 18th integer in the array, you would need to do the
following:

then everything works fine. In this case, the structure does not contain a
pointer and the GEP instruction can index through the global variable,
into the first field of the structure and access the 18th i32 in the
array there.

In this example, idx1 computes the address of the second integer
in the array that is in the structure in %MyVar, that is MyVar+4. The
type of idx1 is i32*. However, idx2 computes the
address of the next structure after %MyVar. The type of
idx2 is { [10 x i32] }* and its value is equivalent
to MyVar + 40 because it indexes past the ten 4-byte integers
in MyVar. Obviously, in such a situation, the pointers don't
alias.